We have crystallised the extracellular domain of the beta subunit of the interleukin-5 receptor. This is an important cytokine receptor that has been implicated in the pathogensis of Asthma and certain parasite infections. In order to facilitate derivativisation, a number of cysteine mutants have been produced. Some of the mutant crystals grow only to a small size. By using synchrotron radiation we should be able to (a) increase the resolution of data from our known derivative, (b) maximise the anomalous signal from the mercury atoms (c) collect data rapidly before icing becomes a problem, and (d) collect data from crystals of other potentially derivative mutants which do not grow large enough to diffract on our conventional source. BioCARS Station 14-BM-D was used.